Global navigational satellite systems (GNSS) include the global positioning system (GPS), the Galileo positioning system, and the global orbiting navigational satellite system (GLONASS). GNSS-based navigational systems are often utilized by military and civilian naval, ground, and airborne vehicles or platforms for navigation, targeting, and positioning applications.
In a GPS navigational system, GPS receiver units typically receive satellite or coded GPS signals from a set of twenty-four (24) Navstar satellites deployed in 12-hour orbits about the earth and dispersed in six orbital planes at an altitude of 10,900 nautical miles in half geosynchronous orbits. A GPS constellation can include more or less than 24 satellites. The position of the GPS satellites is controlled and monitored by the Department of Defense (DoD). GPS satellites continuously emit coded GPS signals.
The GPS signal contains timing information that allows a user to determine the time elapsed for the GPS signal to traverse the distance between the GPS satellite and the user (the platform). By knowing the time the GPS signal left the GPS satellite, the time the GPS signal arrived at the user, and the speed of the GPS signal, the user can determine the distance from itself to the GPS satellite. By knowing the position of the GPS satellite (ephemeris data), and the distance from itself to a set of typically four GPS satellites, the user can successfully calculate its own position.
The GPS signal emitted by the satellites contains L-band carrier components at the transmitted frequencies of 1.575 GHz (L1) and 1.2276 GHz (L2). The L1 carrier component is phase shift keyed (PSK) modulated by two orthogonal pseudo-random (PRN) ranging codes, a precise P(Y) code at a chipping rate of 10.23 MHz and a course acquisition (C/A) PRN code at a chipping rate of 1.023 MHz. Navigation data at 50 bits per second is modulo-2 added to each ranging code. The PRN ranging codes provide timing information for determining when the GPS signal was broadcast. The data component provides information, such as, the satellite orbital position. The L2 carrier is similar to the L1 carrier except that it contains either one but not both simultaneously PSK modulates P(Y) and C/A codes. A military GPS receiver can process both P(Y) and C/A signals. A civilian receiver can only process C/A and P signals.
Position determination using a conventional GPS receiver is well known in the art. In conventional GPS, a receiver makes ranging measurements between an antenna coupled to the receiver and each of at least four GPS satellites in view. The receiver makes these measurements from the timing information and the satellite orbital position information obtained from the PRN code and data components of each GPS signal received. By receiving four different GPS signals, the receiver can make accurate position determinations.
The receiver acquires the satellite signals after down conversion by a direct injection local oscillator (LO). The LO is referenced and locked to a crystal oscillator. The downconverted signal is quantized and digitally processed to determine PRN code position and the data component, hence, to calculate position information.
Satellite spoofer systems, such as GPS satellite spoofers, can deny access or degrade the navigation or positioning performance of satellite positioning equipment, such as GPS user equipment. A spoofer is a device that replicates or mimics a satellite positioning signal, such as a GPS signal, and transmits this replicated signal (e.g., a false or spoofer signal) with the intent of denying access to the real, true positioning signal. Conventional GPS receivers are susceptible to interference with the acquisition and tracking of real satellite signals and to errors in the positioning solution or navigation solution due to spoofer signals. This can cause degradation to the receiver's ability to acquire satellites and can degrade the receiver's calculated position solution to the extent that the position solution becomes unusable. If this degradation is not detected, it could jeopardize the mission for which the receiver is being utilized. A spoofer can interfere with the normal operation of a GPS receiver to varying extents. To the casual user, this error may or may not be significant, but to the precise positioning user, this position error may jeopardize the mission.
Thus, there is a need for a system that can quickly locate a source of a replicated signal or spoofer signal. There is also a need for a system that can differentiate between true signals and replicated spoofer signals. Further, there is a need for a positioning receiver that will receive a replicated spoofer GPS signal and that may also receive a true GPS signal. Further still, there is a need for a GPS receiver that can acquire and track a spoofer signal or a multitude of spoofer signals having the same or different identifications. Further, there is a need for a system which can collect the spoofer signal information from multiple sources. Further, there is a need for a system which can provide anti-spoofing information in the form of spoofer locations. Yet further, there is a need for a system which can determine an exact location of a GPS satellite spoofer to assist in the targeting, removal, and tracking of spoofing devices.